While the thrust capability of tunnel boring machines has increased greatly in the past ten years, there still remains a need for tunnel boring machines having significantly greater thrust capability. The difficulty of increasing thrust capability is attributable to the relatively low load capacity of the machines' thrust bearings which are like the weak link in a chain in that all thrust generated by today's machines is transmitted to the cutter head through the thrust bearings. Because of this problem, a lot of attention has been directed towards increasing the thrust bearings' load capacity. For example, research has been conducted into making the bearings out of higher quality steels, steepening the cone angles of the bearings and using three roller race bearing designs. None of this research, however, is expected to significantly increase the bearing's load capacity. Twenty to forty percent increases are possible but 100 percent increases are believed to be out of the question. The bearings' load capacity problem also cannot be solved by simply making the bearings larger. Those who have ever squeezed past the bearing housings on hard rock tunnel boring machines know that larger main thrust bearings cannot be installed due to space limitations.
An object of the present invention is to significantly increase a tunnel boring machine's thrust capability without increasing the size of the machine's thrust bearings.
Another object of the present invention is to retrofit existing tunnel boring machines to increase the machine's thrust capability.
Yet another object of the present invention is to increase the thrust capability of existing machines at a low cost without having to replace the machine's existing thrust bearings.
Yet another object of the present invention is to provide a tunnel boring machine that does not have to transmit thrust to the cutter head through the machine's thrust bearings.
Yet another object of the present invention is to provide a tunnel boring method for increasing the speed of tunnel boring.